This invention relates to a method for etching titanium nitride on semiconductor substrates.
Conductive features are used to electrically connect devices formed on semiconductor substrates. The conductive features typically comprise (i) a bottom barrier layer, (ii) an electrically conductive metal-containing layer, such as an aluminum alloy, at the middle of the feature, and (iii) a top antireflective layer, such as titanium nitride. An insulative oxide layer, such as a silicon oxide layer, is deposited on top of, and between the features, to electrically isolate the features. Apertures are etched through the insulative oxide layer and the titanium nitride layer to form holes. The holes are filled with conductive metal to form vertical electrically conductive interconnects, commonly known as vias.
The apertures through the insulative oxide and titanium nitride layers are etched in a multistep process. First, the insulative oxide layer is etched using reactive ion etching processes with fluorine-containing etchant gases. After etching of the insulative oxide layer, a wet chemical etching process is used to etch the titanium nitride layer of the features, to expose the conductive metal-containing layer in the features. Etching of the titanium nitride layer is necessary for the conductive interconnect to electrically contact the conductive metal-containing layer.
There are several problems with the multistep etching process. First, the reactive ion etching processes used to etch the oxide layer often have an inadequate etching selectivity ratio. The etch selectivity ratio is defined as the ratio of the etch rate for the etched layers to the resist etch rate. When both the insulative oxide and titanium nitride layers are etched, the combined etch rate for the insulative oxide and titanium nitride layers must be greater than the resist etch rate, in order for the resist layer to effectively protect the underlying oxide and titanium nitride layers. Excessive etching of the resist layer is also undesirable because it can cause excessive deposition of polymeric resist etchant byproducts on the substrate and on the walls of the etching apparatus. Excessive quantities of such deposits are difficult to remove. Consequently, the etching selectivity ratio of the reactive ion etching process is preferably at least about 3, and more preferably at least about 4.
Another problem with current techniques relate to the wet chemical etching process used to etch the titanium nitride layer. To effect the wet chemical etching process, the substrate must be removed from the reactive ion etching apparatus and transferred to a wet chemical etching station. The transfer operation limits process throughput efficiency. Furthermore, when the substrate is exposed to the atmosphere during transfer, the etched layers on the substrate can corrode forming contaminants on the substrate. Also, wet chemical etchant processes often leave contaminant chemical residues on the substrate. These contaminants are only discovered in the final processing stages when the fully processed wafers are worth between $850,000 to $100,000, and often the entire wafer must be scrapped.
Thus, there is a need for a reactive ion etching process capable of etching the titanium nitride layer on the substrate. Such a process can provide greater process efficiency and can increase integrated circuit chip yields, compared to existing wet chemical etching processes. It is also desirable for the etching process to have a high etching selectivity ratio, and be amenable to mass production of circuit chips in conventional etching apparatus. It would be even more desirable for the etching process to be capable of removing both the insulative oxide layer and the titanium nitride layer on the substrate.